Cumene is an important intermediate in the chemical and polymer industries, with global cumene production currently exceeding twelve million metric tons annually. The majority of all cumene manufactured in the world today is used for the production of phenol. The demand for phenol for the manufacture of Bisphenol-A and subsequently polycarbonates is accelerating, owing to the broadening applications of polycarbonates in the electronic, healthcare, and automobile industries.
The rapid growth of cumene, phenol and Bisphenol-A production, however, has caused some concerns related to the imbalance of the acetone byproduct produced from the phenol plant. Thus, acetone and phenol are produced at an approximately 1:1 molar ratio from cumene, but are used at an approximately 1:2 molar ratio in the downstream Bisphenol-A production process. The excess acetone that is not used in the production of Bisphenol-A has caused some concern from phenol producers in that it may create a supply-demand imbalance and disrupt the economics of the phenol production business.
In addition, cumene is typically produced by reacting benzene and propylene under liquid phase or mixed gas-liquid phase conditions in the presence of acid catalysts, particularly zeolite catalysts. The resultant need to locate integrated cumene/phenol plants near a source of propylene has become an important issue with producers. Thus, in today's olefins market, there is also a supply-demand imbalance in the supply of propylene produced from conventional sources, such as ethylene plants, due to the availability of feedstocks that favor the production of propylene. This imbalance has forced phenol producers to build their plants closer to feedstock supplies rather than to product outlets.
Numerous research efforts have been directed at solving the acetone imbalance and propylene supply issues described above by recycling the excess acetone produced in the phenol plant to produce cumene. For example, U.S. Pat. No. 5,015,786 teaches a process for preparing phenol, comprising the steps of: (a) alkylating benzene with isopropanol using a zeolite catalyst under liquid phase conditions to synthesize cumene, (b) oxidizing the cumene from step (a) with molecular oxygen into cumene hydroperoxide, (c) subjecting cumene hydroperoxide to acid cleavage to synthesize phenol and acetone, and (d) hydrogenating the acetone from step (c) with hydrogen gas under liquid phase conditions into isopropanol which is recycled to step (a).
One problem involved in manufacturing cumene by the alkylation of benzene with isopropanol over a zeolite catalyst is that one mole of water is produced for every mole of isopropanol consumed during the reaction. Not only does this require the provision of significant downstream treatment facilities to remove water from the product but also most zeolites show decreased selectivity and increased aging in the presence of excessive amounts of water. It is therefore desirable to maintain the water concentration at the outlet of the isopropanol alkylation reactor at about 1 wt %. As disclosed in, for example, U.S. Pat. No. 6,512,153, this is conveniently achieved by controlling the rate of alkylation and recycling part of the reaction effluent after drying to remove the water by-product.
Another problem encountered in using isopropanol produced from the excess acetone from a phenol plant in the production of cumene is that the acetone tends to contain significant quantities of nitrogen impurities which carry over into the isopropanol intermediate product. Such impurities act as poisons to the zeolite catalyst employed in the downstream alkylation step and so must be removed or reduced to very low levels. However, attempts to remove these impurities from the acetone and isopropanol feeds with conventional solid acid adsorbents have proved to be only marginally effective due to the molecular polarity of the acetone and isopropanol, which competes with the adsorption of the polar nitrogen compounds. Also, the high water solubility of acetone and isopropanol eliminates the use of water washing, which is also commonly employed to remove nitrogen compounds from hydrocarbon streams.
In accordance with the present invention, it has now been found that, when isopropanol is produced from acetone and is used in the alkylation of benzene to produce cumene, the deleterious effects of any nitrogen impurities in the isopropanol can be effectively ameliorated by treatment of the dried recycle stream to the alkylation reactor. In addition, by effecting the treatment on the dried recycle stream, the removal of the nitrogen impurities can be achieved by adsorption on acidic solid adsorbents, such as molecular sieves or acidic clay, and/or by treatment with an acidic water solution, such as dilute sulfuric acid. Alternatively, the isopropanol can be mixed with fresh benzene and the resulting mixture treated to remove the nitrogen impurities. In this case, the dilution effect of the benzene on the polarity of the isopropanol is found to be sufficient to allow the use of solid acid adsorbents in the nitrogen removal. Moreover, treatment of the benzene/isopropanol mixture can be used to remove nitrogen and other impurities from the fresh benzene.